Adverse drug interactions secondary to induction of drug detoxification genes by ligand activated PXR is an important clinical problem leading to decreased drug efficacy. Despite our ability to use rapid in vitro screens to identify compounds that activate PXR, there is still no way to predict the extent of drug interaction in individual patients. We believe that human variation in PXR expression contributes to the interindividual differences in induction of CYP3A. Moreover, the growing list of drug detoxification genes induced by PXR further justifies the need to determine what is responsible for variable PXR expression. Our prior studies have revealed that sequence variation (SNPs, single nucleotide polymorphisms) in the PXR coding region is not significantly contributing to human variation in PXR expression or human variation in induction of the CYP3A target gene. Our current research is designed to further elucidate the cellular and molecular mechanisms regulating PXR expression. Our prior studies have revealed that there are multiple isoforms of PXR that differ in their amino terminus and in the ligand binding domain. Our current research in Aim 1 is designed to discover the extent to which PXR with different amino termini and different ligand binding domains are expressed in human tissues and whether these PXR splice variants have altered function. We hypothesize that these variant PXRs fine tune PXR activation creating an additional layer of regulation, and ultimately influence the CYP3A inductive phenotype. Towards the goal of understanding factors regulating PXR expression Aims 2 and 4 will determine the role of liver enriched transcriptional factors in regulation of human PXR transcription and PXR expression in human liver. In order to visualize PXR transcriptional activity in mouse tissues, in Aim 3 we will develop a transgenic mouse with the PXR-promoter-driving a luciferase reporter. Bioluminescent imaging through the skin of the mouse will monitor in real-time PXR transcription under the most informative biological conditions-i.e., in a living intact animal. This unique and powerful whole body imaging approach will allow us to visualize PXR transcription in multiple tissues under different physiological and drug treated environments. In total this proposal describes a comprehensive set of experiments to determine how PXR is regulated.